PURPOSE: To prevent missing of data in an OA device due to trouble in power supply system, by connecting a specific machinery in the OA device with receptacles having current detecting functions and connecting other machinery with other receptacles.

CONSTITUTION: A tap T is connected with first and second power supply lines leading out from distribution boards 3, 4. Three receptacles C1∼C3 are provided for the tap T. Coil RX in a current relay is connected with the receptacle C1. Normal open contacts MS1, MS2 of the current relay are connected with the receptacles C2, C3. When a switch PS1 of an OA device 1 is turned ON, relay coil RX is energized to close the contacts MS1, MS2. Consequently, power is conducted through the receptacles C2, C3 and an OA device 2 can be operated by turning a switch PS2 of the OA device 2 ON.